(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having a carburetor fuel system.
(2) Description of the Related Art
At present, a three-way catalyzer is used to convert three noxious gas components contained in an exhaust gas of an engine into innocuous gas components. Namely, noxious carbon monoxide (CO) and hydrocarbon (HC) are oxidized and nitrogen oxides (NO.sub.2) are deoxidized simultaneously by the three-way catalyzer into carbon deoxide (CO.sub.2), water vapor (H.sub.2 O), and nitrogen (N.sub.2) respectively. It is known that the cleaning capacity of the three-way catalyzer becomes greatest when the air-fuel ratio is at a stoichiometric air-fuel ratio.
In the prior air-fuel ratio feedback control system in an internal combustion engine having a carburetor fuel system, the O.sub.2 sensor is arranged in an exhaust system and located close to a combustion chamber of the engine, i.e., the sensor is positioned at the gathering point of an exhaust manifold located upstream of the three-way catalyzer, and an air bleed pipe is connected to a fuel passage between a nozzle and a fuel reservoir of the carburetor. The air-fuel ratio in the prior internal combustion engine having a carburetor fuel system is controlled by adjusting the amount of air flowing through the air bleed pipe and mixed with the fuel in accordance with a signal output from the O.sub.2 sensor. That is, when the signal output from the O.sub.2 sensor indicates a lean state of the engine, the amount of the air bleeding is decreased to enrich the air-fuel ratio, and when the engine is in a rich state, the amount of air bleeding is increased to make the air-fuel ratio leaner.
However, to stabilize the running state of a cold engine, and to improve the driveability in an acceleration state and a heavy load state of the engine, the above mentioned air-fuel ratio feedback control is interrupted when the engine is in a cold state, in an acceleration state, or in a full or heavy load state.
This interruption of the air-fuel feedback control brings the air-fuel ratio to a base air-fuel ratio, which is predetermined to be on the rich side of the air-fuel ratio when the engine is in the cold state, in the acceleration state, or in the full or heavy load state, thereby increasing the HC and CO emissions. As a result, the driveability at the acceleration state and the full or heavy load state of the engine becomes worse and the fuel consumption is raised.